Optical discs represented by DVDs (Digital Versatile/Video Discs) are commonly used media capable of storing a large amount of digital data such as AV (Audio Video) data and computer data. For example, high-quality video which is over two hours in length is recorded on read-only optical discs for sale. In order to prevent such digital copyrighted works from being illicitly copied onto other recording media, a method known as content encryption is employed (See “Nikkei Electronics” Nov. 18 (1996): 13˜14).
In the above method, compressed digital content such as movies is encrypted by the use of three-layered secret keys (title key, disc key, and master key) and recorded in a user information area accessible by a user. Of these secret keys, the master key, which is most important of all, is notified only to a licensed authorized manufacturer, while the disc key and the title key required for each DVD and title are encrypted on the basis of the master key, and stored in a control information area inaccessible by a user. This prevents a user's access to the secret keys required for decryption, and therefore an unauthorized copying such as by means of file copying cannot be carried out.
However, the above method allows encrypted content to be decrypted and reproduced by ordinary reproduction apparatuses, when the whole contents of a recording area including the control information area in which the secret keys are recorded, is illicitly copied onto another optical disc.
In response to this problem, there is a method, as shown in FIG. 1, for recording sub information onto an optical disc 2000, on which main information such as digital content is stored, by means of phase modulation to displace standard edge positions of recording marks 2001 that constitute the main information in the track direction by a minute amount (See Japanese Laid-Open Patent application publication No. 2001-357533). FIG. 2 is a timing chart showing major signals that are used in the above method at the time of recording. According to this method, a modulated channel signal C6 is obtained by means of phase modulation in which a channel signal C2 which constitutes main information is advanced by a minute amount when a PE modulated signal C5, which is obtained by performing PE modulation on a result of carrying out exclusive OR between a random number sequence C4 and sub information to be recoded, is “H”, whereas the channel signal C2 is delayed by a minute amount when the PE modulated signal C5 is “L”. According to such modulated channel signal C6, main information and sub information are recorded on an optical disc as modulated recording marks C7. In other words, main information and sub information are recorded onto an optical disc by means of phase modulation in which an edge of a standard recording mark C3, which constitutes the main information, is advanced by a minute amount in the track direction when the PE modulated signal C5 is “H” and is delayed by a minute amount in the track direction when the PE modulated signal C5 is “L”. Accordingly, since sub information superimposed on the edges of recording marks cannot be copied even if the whole contents of a recording area including the control information area is illicitly bit-copied onto another optical disc, it becomes impossible for an ordinary reproduction apparatus and the like to decrypt an illicitly copied optical disc, by recording information including secret keys as sub information.
However, the above existing technique is not capable of verifying whether sub information is normally recorded or not, although it is capable of verifying whether main information is normally recorded or not. Thus, there might arise a problem that even if main information is normally recorded, sub information which is supposed to be superimposed in the same area, has failed to be recorded. For example, in the case where a content decryption key is recorded as sub information, when an area is reproduced in which only main information is recorded normally and in which sub information failed to be recorded normally, it is impossible to decrypt the encrypted content because the sub information cannot be reproduced normally.
Furthermore, in the existing technique, defect management is conducted, for example, in accordance with the linear replacement algorithm and the like, which is one of the defect management processes intended for DVD-RAMs, when an area is judged to be a defective area where main information cannot be recorded normally. In the linear replacement algorithm, when a defective area is found at the time of verifying recorded information, such area is relocated to a spare area which is provided in advance within an optical disc user area. Then, the respective top sector numbers of such defective area and its corresponding replacement area are registered and managed as an entry pair in a secondary defect list (SDL) which exists in a lead-in area and a lead-out area of an optical disc. Moreover, the above defective area and replacement area are provided in a unit of ECC block (16 sectors) and relocation processing is performed on an ECC block basis.
FIG. 3 is a conceptual diagram showing an existing linear replacement processing. As shown in FIG. 3A, inside an optical disc are: a user area 1701 in which main information is stored by forming optically-readable recording marks and in which sub information is stored by displacing their recording mark edges by a constant minute amount; a spare area 1702 intended for relocation used when the user area 1701 is a defective area; and, as shown in FIG. 3B, a secondary defect list (SDL) 1703 for managing the respective top sector numbers of a defective area and a replacement area as an entry pair when such defective area is relocated to the replacement area (which is within the spare area 1702). For example, suppose that a first defective area 1704 and a second defective area 1705 are judged to be defective as a result of performing verification to see if main information is recorded normally or not. In such a case, both pieces of main information recorded in the first defective area 1704 and the second defective area 1705 are relocated to a first replacement area 1706 and a second replacement area 1707 inside the spare area 1702, respectively.
However, the existing defect management processing described above has the problem that only main information is relocated and recorded in a replacement area when an area is judged to be a defective area where main information cannot be normally recorded, causing sub information superimposed on such main information to be unable to be replaced and recorded.
Moreover, since only a reproduction error related to main information is detected in the existing technique at the time of reproducing an area where main information and sub information are recorded, reproduction processing keeps going on even when sub information has become uncorrectable or subject to error correction.
The present invention has been conceived in view of the above problems, and it is an object of the present invention to provide an optical disc defect management method, an optical disc recording apparatus and an optical disc reproduction apparatus capable of reproducing main information and sub information in a stable manner even when not only a defective area where the main information cannot be normally recorded is found, but also a defective area where the sub information superimposed on such main information cannot be normally recorded is found.